Thermal chemical reactions are those chemical reactions that produce (exothermic) or consume (endothermic) heat. Examples of thermal chemical reactions include hydrocarbon conversion reactions such as steam reforming, water-gas shift reactions and combustion. These well-known reactions are usually carried out in the presence of a catalyst at temperatures up to about 1300° C. Because the intrinsic kinetics of a thermal chemical reaction can be much faster than the heat transfer rate between the reaction vessel and the thermal sink, source or environment, the actual rate of product production (i.e., the observed rate) is slower than the intrinsic rate. Intrinsic kinetics means the rate at which products could theoretically be formed at the catalyst surface.
Limited production rates may result from longer residence time which is typically seconds to minutes in conventional thermal chemical reaction vessels. As it is conventionally defined, residence time is equal to the volume of the reaction zone divided by the inlet volumetric flow rate of reactants at the reaction system's temperature and pressure. The reaction zone is the total volume of the catalyst and surrounding area through which reactants and products flow.
An example of these limited production rates can be seen in the water gas shift reaction which is conventionally carried out in fixed bed reactors. In the water gas shift reaction, carbon monoxide and water are converted to carbon dioxide and hydrogen. Conventionally, this reaction suffers from multiple-second residence times (a kinetic impediment) when carried out in fixed bed reactors. Theoretical kinetics suggests that residence times on the order of milliseconds could, theoretically, be obtained. There are two kinetic retarding aspects to conventional reactors. The first is a diffusion limitation as reactants diffuse into and out of a catalyst-bearing porous pellet and the second is a heat transfer limitation which is a combination of heat transfer parameters (thermal conductivity and length) of catalyst supports and overall reactor geometry (shape, size, and distance to the external heat exchanger). Because the water gas shift reaction is critical to a multi-reactor fuel processing system that supports distributed energy production through the use of a fuel cell, there is a need for a smaller, faster water gas shift reactor.
Another example of a thermal chemical reaction is in the conventional methane steam reforming reactor which produces synthesis gas at an average residence time of several seconds and with an effectiveness factor of 0.01 to 0.05 as reported by Adris, A., Pruden, B., Lim, C., J. Grace, 1996, “On the reported attempts to radically improve the performance of the steam m thane reforming reactor,” Canadian Journal of Chemical Engineering, 74, 177–186. In a typical industrial operation, the methane to steam ratio is run at 3:1 to prevent coke formation. Efforts to improve heat transfer between the reaction vessel for this endothermic reaction and the thermal source have made only modest improvements in product production rate.
Thermal reactions have long been, and continue to be, conducted in huge volumes on production scales requiring very large capital investments, typically greater than $100 million. Not surprisingly, there have been extensive efforts, over a long period of time, aimed at improving the speed and efficiency of these reactions. Despite these attempts, there remains a need for a method and apparatus that increase the heat transfer rate between the reaction vessel and the thermal sink or source and thereby approach the theoretical intrinsic kinetic rate of reaction and production.